The present invention relates to a method for the automatic regulation of the high frequency output power of a high frequency amplifier including a regulating circuit wherein a voltage which is dependent on the high frequency (HF) output power is obtained and used to generate a regulating voltage with the aid of a reference voltage, and this regulating voltage is used to produce a control voltage or a control current which controls the amplifier gain. The present invention also relates to a circuit arrangement for implementing the method according to the invention.
In power amplifiers for amplifying high frequency signals it is desirable to operate in a linear mode at small and medium HF power and to limit the output power at a higher HF output power, with the power level at which the limiting starts being variable according to characteristics OAE and OA'E' shown in FIG. 1. By making available such limiting characteristics, there results the advantage that the current supply device need provide only the power corresponding to the limitation value AE of the HF output power. However, it is necessary for limitation characteristics AE and A'E', to be employed only for mean power, and therefore, for short-time fluctuations of the input power in the lower frequency range of the communications signals or of the high frequency carrier, the natural characteristic OABC and OABD, respectively, of FIG. 1 continues to be applicable as before. This condition must be met by suitably dimensioning the regulating time constant of the corresponding regulating device.
Various circuit arrangements are known for producing the limitation characteristic, examples of which are shown in FIGS. 2 and 3. FIG. 2 shows the conventional ALC (automatic level control) circuit wherein part of the output power produced by high frequency amplifier 1 is coupled out and is rectified in coupler 2, is then compared in a comparator 3 with a reference signal REF, amplified, and finally fed to a controllable attenuation member 4. In this way, the amplitude of the input signal P.sub.1 of high frequency amplifier 1 is influenced in such a manner that the control loop keeps output power P.sub.2 at a constant level. If the input power P.sub.1 is continuously reduced, attenuation member 4 of FIG. 2 will reach its minimum attenuation in the region of characteristic AA' of FIG. 1, so that the control loop is interrupted and amplifier 1 operates according to its natural characteristic OA'A.
The circuit according to FIG. 3 operates in a similar manner. In contradistinction to FIG. 2, the output power of amplifier 1 is here regulated by way of a control electrode, independently of the input power P.sub.1.
The known circuit arrangements for realizing the desired limitation characteristics have the drawback that additional components, primarily high frequency modules, are required for the regulating circuit. These are, for example in FIG. 2, the controllable attenuation member 4 and the coupler with integrated rectifier 2. Added to this are the HF transmission losses of the coupler 2.